1. Field of the Invention
The present invention relates to a vessel for use in hydrothermal syntheses such as one involved in the production of artificial rock crystals.
2. Prior Art
A vessel conventionally used in hydrothermal syntheses is shown in FIG. 4 and consists generally of a vessel body 21, a cover 22, clamps 23, a convection control plate 24, a heater 25 and thermocouples 26. Seeds A and a feed material B in the vessel body 21 are submerged in a strong alkali solution heated with the heater 25. To take the hydrothermal synthesis of artificial rock crystals as an example, the vessel is used at temperatures of 350-400.degree. C. and at pressures of 1,000-1,500 kg/cm.sup.2, so the vessel body 21 must be made of metallic materials that have high strength, toughness and corrosion resistance.
Many of the articles produced by hydrothermal syntheses are intended to be used in such applications as electronics and optics where there is a growing demand for reduction in both size and thickness. When a small and thin strip is to be produced by hydrothermal synthesis, a major cause of deterioration of the product is contamination by foreign substances. A particularly pronounced problem is that the inner surface of the vessel body 21 is corroded by a strong alkali solution to produce an iron compound called "acmite" and that the product is contaminated by Fe+ ions. Various methods have been employed to solve this problem and one of them is to isolate the surface of seeds A. However, this method is not completely satisfactory and the growth of rock crystal is impeded at the protected seed surface to reduce the production rate.
Another approach is to prevent the generation of Fe+ ions per se by either coating the inner surface of the vessel body 21 with a precious metal such as silver, gold or platinum or placing an inner tubular vessel made of these precious metals within the vessel body 21. In the first case where the inner surface of vessel body 21 is coated with a precious metal such as silver, gold or platinum, the adhesion between the precious metal coat and the inner surface of vessel body 21 must be insured by such means as hydraulic expansion of the tubular body or explosive bonding but this involves considerable difficulty in manufacturing the vessel. Further, depending on the construction of vessel body 21, inadequate maintenance of the sheet surface in the sealed area might occur. In the second case where an inner tubular vessel made of a precious metal such as silver, gold or platinum is placed within the vessel body 21, the pressure within inner tubular vessel must be made equal to that working outside so that undue external pressure will not act on this inner vessel. To this end, it is essential that the volume of fluid within the inner vessel be held equal to that outside of the vessel. For these reasons, the two methods described above which are intended to prevent the generation of Fe+ ions have been applicable only to small experimental vessels.